The present invention relates to a device for temporarily blocking the re-measurement of an electric oil-level measurement of a machine.
Particularly the invention relates to a device for temporarily blocking the re-measurement of an electric oil measurement of a machine, particularly an internal combustion engine, it having a measurement probe which is heated by a constant current during a connect time which is commenced by a switch for the placing in operation of the machine, in particular an ignition switch, means for comparing an initial voltage U.sub.O on the measurement probe at the start of the connect time with a measurement voltage at the end of the connect time of a measurement period, a signalling logic which can be controlled as a function of the result of the comparison in order to signal the oil level, and a blocking-time transmitter which, in combination with a control logic, blocks repeated measurement for at least a predetermined period of time after the machine is turned off or again turned on.
With one such known device the oil level of an internal combustion engine is measured in the manner that upon actuation of the ignition key and before the starting of the internal combustion engine the measurement probe is acted on by a constant current and the voltage drop on it is measured and evaluated during different points of time of the connect time. The measurement probe has a relatively high temperature coefficient, in particular a positive temperature coefficient in the case of a measurement probe consisting of nickel/iron wire, so that the voltage drop on the measurement probe increases with an increase in the temperature. At the start of the connect time of the constant current, a first measurement is effected, namely with an initial voltage U.sub.O which is dependent on the ambient temperature and/or the oil temperature. Upon the heating of the measurement probe by the constant current the temperature of the measurement probe changes as a function of the level of the oil, which takes up a larger or smaller amount of the heat. Thus the measurement voltage which is measured at the end of the connect time and compared with the initial voltage U.sub.O is a measure of the oil level. As a function of the result of the comparison, the "warning" or "non-warning" condition of a warning lamp can be brought about via a signalling logic.
It is known that, upon the starting of an internal combustion engine and its further operation, the oil is conducted out of the crankcase sump via ducts to holes on the surfaces to be lubricated. As a result, the oil level in the crankcase sump drops even if a sufficient amount was originally present. Upon the turning off of the engine, the oil gradually flows back into the sump, so that after a very long time an oil level is reached which is a reasonably exact measure of the amount of oil actually present.
Accordingly, the oil level which can be measured upon the starting of the internal combustion engine is not directly a measure of the amount of oil present in the engine, which in particular makes it difficult to effect a properly dosed refilling of the oil. In this connection, it must be borne in mind that, when the internal combustion engine is turned off, as a rule one does not have all the time necessary in order to wait so as to be certain that all of the oil has returned and thus obtain a measurement result which is definitely valid. In addition to this, there is the fact that the oil level of the internal combustion engines in automotive vehicles is frequently measured by the driver and thus not always by technically interested and/or trained persons. If special measures are not taken, it must therefore be feared that too much oil will be added since the signalling device erroneously signals too small an amount of oil in the engine, as a result of the oil which is present in the ducts and has not flown back, or else the driver no longer needs a warning for the reasons described and fails to refill even if it should be necessary. The disadvantageous consequences of too small an amount of oil for lubrication and cooling in internal combustion engines are well known, as are also the disadvantageous consequences of too large an amount of oil, the excess of which, in the best of cases, is thrown out unused by the engine.
Therefore, in one known device for the electric measurement of oil, a device for blocking re-measurement has already been provided which is intended to take into account the return flow of the oil into the crankcase sump. The device for blocking re-measurement has the result that for a fixed predetermined blocking time after an immediate restart of the engine no new measurement can, for the time being, take place in order to permit the oil to return in the meantime.
However, it has been found that this device for blocking re-measurement can operate satisfactorily only for given parameters of ambient temperature and oil temperature since the viscosity, and thus the return time of the oil, is, as is known, dependent on temperature. This has the result that in hot weather and with a hot engine the oil flows rapidly back into the crankcase while the same process takes a relatively very long period of time in very cold weather and when the engine has been operated only for a short time.